High-speed signals have a tendency to travel on the outer edge, or “skin”, of a conductor. Thus, the cross-sectional area of the conductor that is used to transmit the signals is reduced. Because less conductor is used, the transmission path is, effectively, more resistive. The resistance here is referred to as the “skin resistance”. Losses in the signal that results from the skin resistance are referred to as “skin losses”.
Skin losses become more prevalent as signal frequencies increase, and can have various deleterious effects on the signal. For example, skin losses can cause attenuation in the signal, which effectively results in a narrowing of signal pulses. In a square wave signal, such as a digital signal, the attenuation can cause a rounding of the signal. For example, as shown in FIG. 1, skin losses can transform original, complementary square-wave digital signals 5 into signals 7 having rounded edges. This results in a narrowing of pulse widths in the signals, thereby adversely affecting timing. In some cases, amplitude attenuation can be significant enough to prevent the signals from reaching a threshold required to register a change from a logic zero to a logic one.
Problems resulting from skin loss can occur in automatic test equipment (ATE). In this context, ATE is an automated, usually computer-driven, system for testing devices, such as semiconductors, electronic circuits, and printed circuit board assemblies. A device tested by ATE is referred to as a device under test (DUT).
ATE is capable of providing different types of signals to a DUT. Among these signals are test signals, which are used to test the DUT. The test signals may include analog signals and digital signals used to test and/or program the DUT. Heretofore, ATE provided a fixed loss compensation to counteract skin losses that occurred during transmission of signals between the ATE and the DUT. One problem with fixed loss compensation, however, is that it does not take into account that different signal transmission paths have different lengths, resulting in different amounts of loss. Circuit board traces, for example, can account for the differing lengths of a transmission path. Fixed loss compensation can under-compensate for skin losses and, in other cases, it can over-compensate for skin losses.